In electrophotography, multicolor images having resolution and other qualities comparable to those of silver halide photography have been produced in the laboratory. One reason such systems have not been commercially practical is they have generally required liquid developing for high quality. However, recent advances in fine particle dry toners have made low-grain, high-resolution images feasible with dry systems.
U.S. patent application Ser. No. 07/405,258, filed Sept. 11, 1989, TONER FIXING METHOD AND APPARATUS AND IMAGE BEARING RECEIVING SHEET, to Rimai et al, discusses a problem with such high resolution dry images that when they are put through an ordinary roller fuser they both spread, losing resolution, and exhibit a substantial relief image according to the varying thickness of toner layers in the image. The Rimai et al application suggests using a hard ferrotyping belt to embed the toner in a heat softened thermoplastic layer. The combination of relatively high pressure and the heat softened thermoplastic layer both substantially embeds the toner in the layer substantially reducing the relief without spreading the image and also applies a gloss to the image highly desirable in some applications.
U.S. patent application Ser. No. 07/409,194, filed Sept. 19, 1989, METHOD AND APPARATUS FOR TREATING TONER IMAGE BEARING RECEIVING SHEETS, Baxter et al, deals with texturizing or adding gloss to a toner image-bearing receiving sheet generally of the type described in the Rimai et al application having a toner image on a thermoplastic layer. This application suggests that there are certain advantages if the thermoplastic layer is softened primarily from heat originating in a roller contacting the back of the receiving sheet rather than one contacting the thermoplastic layer itself. This approach eliminates localized areas of high temperature on the surface of the thermoplastic that may encourage offset of the thermoplastic onto the member contacting it. In the Baxter et al preferred embodiment, the image is first fixed using the smooth ferrotyping belt of the Rimai et al application and then texturized with an unheated texturing roller contacting the image with the receiver backed by a heated roller. Good results were obtained in reducing relief and in adding texture without release oils.
The processes in the above two applications are done without the use of fusing oils because fusing oils leave image defects that are unacceptable with photographic quality prints.
U.S. Pat. No. 4,639,405, issued Jan. 27, 1987 to H. G. Franke, shows a post-treatment step to add gloss to a toner image carried on a paper after ordinary fusing. The fixed image bearing paper is dried and then pressed between a pair of heated rollers which increase the gloss of the image. At least one of the rollers has a resin coating to provide some width of nip to aid in heat transfer. A purpose for the drying step is to prevent blistering from steam escaping around the nip when coated paper is used as a receiving sheet.
U.S. Pat. No. 4,780,742, issued Oct. 25, 1988 to Takahashi et al, shows a method of increasing the gloss of a fixed toner image by coating it with a thin sheet in the presence of heat and pressure. The thin sheet packs the image and fuses it together, increasing gloss and removing surface roughness. The sheet is cooled and peeled off. The image appears to be fused on top of the support which has a principal object of providing less scattering for color images on transparencies.
European Patent Application 0301585, published Feb. 1, 1989, shows a glazing sheet used to increase the gloss of either a toner image on a paper backing or a dye and developer in a thermoplastic coating. The glazing sheet is pressed against the paper sheets with moderate pressure and the dye-thermoplastic sheets with substantial pressure. The glazing sheet can be either smooth for a high gloss or dull for a low gloss finish. In one embodiment the glazing sheet has both high and low gloss sections that can be selected.
U.S. Pat. No. 4,337,303, issued June 29, 1982 to Sahyun et al, suggests a method of thermal transfer involving bringing a receiving sheet having a thermoplastic coating into contact with fine toner images in the presence of sufficient heat to soften the thermoplastic coating. The toner is said to be encapsulated by the thermoplastic coating under moderate pressure.